It is often desirable to obtain measurements of the distance from a known source to an object, or to points on the surface of an object. In certain applications, such distance measurements are subsequently utilized to provide information concerning the shape of the object or provide a definition of its surface plane With regard to distance measurements, it is known to utilize a plurality of microwave frequencies for determining the distance between a pair of active instruments. Signals having each of the different microwave frequencies are transmitted from a first instrument to a second instrument, which is located at some distance from the first instrument. The second instrument receives and sends back signals having the frequencies of the transmitted signals. In this known system, a mathematical technique is employed that relies on the time between zero crossings of the transmitted and received signals of the same frequency. The mathematical technique involves solving a number of equations to determine certain parameters. The optimum values of such determined parameters are then utilized to determine the distance. There is no determination of a coarse distance between the two instruments and a large number of different signal frequencies are employed so that the hardware used to make the distance determination is very expensive. In another known system, a single frequency signal is used to determine the distance between a source and an object. In this system, a coarse distance measurement is made by relying on the propagation delay between the transmission of the signal and the reception thereof. Additionally, a fine distance measurement is made utilizing the same frequency by relying on zero crossing information associated with the single frequency signal. There is no solving of a number of equations to determine the optimum values of parameters for use in determining the distance.
In addition to these known techniques, a number of patents have been granted which disclose measuring systems that use two or more signals of different frequencies in order to determine the distance to a target or an object. U.S. Pat. No. 4,282,589 to Evetts, et al., issued Aug. 4, 1981 and entitled "Correlation Ranging," relates to a ranging process that transmits and receives ultrasonic signals of different frequencies. The transmission of each signal at a particular frequency is separated from the transmission of other signals having different frequencies by means of a time delay. The apparent purpose of the disclosed process is to enhance return signal detection by distinguishing the return signals from noise and any other extraneous signals. In U.S. Pat. No, 4,259,734 to Harmel, issued Mar. 31, 1981 and entitled "FineResolution, Water-Depth Measuring System," a water depth measuring system is described. In making measurements, two frequencies are utilized to assure that the depth recorder is not confused due to the transmission of a pulse before receipt of the echo of a previous pulse. That is, because at least two frequencies are used, the current and prior pulses can be distinguished. U.S. Pat. No. 3,064,234 to Barrett, issued Nov. 13, 1982 and entitled "Sonar System," relates to an ultrasonic system in which a number of different transducers are employed, with each being designed to oscillate at a different carrier frequency. The purpose noted in the patent for using a number of different frequencies is to eliminate noise from the returned signals. Apparently, signals returning from the target will add together while the noise signals will cancel because they vary in phase and amplitude.
Although many systems have been proposed or developed for measuring distances using ultrasonic signals, it would be advantageous to provide a distance measuring apparatus that rapidly and accurately determines the distance between a source and an object or target and be relatively inexpensive. Preferably, it would be desirable to provide a system that is able to accurately measure a number of points found on the object and rapidly determine the distance between each point and the known source. Even though it is known to employ a number of different frequencies and a mathematical technique for determining the optimum value of a parameter from a number of values, that known system did not utilize ultrasonic signals and an instrument that provided return signals was required, not a passive target or object. Most significantly, the processing time involved in this prior art technique was considerable and did not involve the determination of a coarse distance utilizing one of the plurality of frequencies. The present invention is directed to overcoming such disadvantages while providing an accurate and rapid method for determining the distance between a transducer and one or more points located on a passive object.